Plants, herbivores and communities

Rangeland dynamics in the Trans-Himalaya

More than half of the earth’s land surface is comprised of grazing ecosystems, aimed at sustaining populations of large herbivorous mammals, both wild and domestic. However, the goals of conserving wild species are often inconsistent with those of animal husbandry.

Grazing systems

More than half of the earth’s land surface is comprised of grazing ecosystems, aimed at sustaining populations of large herbivorous mammals, both wild and domestic. However, the goals of conserving wild species are often inconsistent with those of animal husbandry. There are numerous examples where wild species have declined with the progressive intrusion of domestic ones.

The Trans-Himalayas represent a vast rangeland system where a large fraction of the original native species assemblages continues to survive alongside a diversity of livestock. However, the landscape is punctuated with instances of local extinctions. At a local scale, (e.g. at the scale of individual valleys or catchments), there are many instances where one or more wild species have gone extinct in the recent past. Our previous work has shed light on the mechanisms by which livestock can drive this process. However, a critical question remains to be answered – how does the ecosystem as a whole fare, with a reduced number of species?

Plant-herbivore co-evolution

For ecosystems that have evolved with a certain assemblage of herbivores and plants, any sudden extinction event can cause imbalances in the co-evolved pathways that the system would normally follow. An ecosystem’s performance is adjudged by the functions it carries out. For instance, among the most familiar attributes are the primary and secondary productivity i.e., the ability to provide food. Other functions include nutrient-cycling that keep soils fertile, hydro-dynamics that run the water cycle and so on. Loss of species can potentially have serious repercussions on ecosystem functions.

So we must ask, what are the consequences of losing native species and replacing them with livestock on ecosystem processes? This is the broad question we have set out to answer in this project.

Species diversity across several taxa ranging from plants to vertebrates is
reported to decrease with altitude, or to show a mid-elevation peak in mountain
systems. Plant biomass availability for herbivores is similarly expected to decline
with altitude as temperature becomes limiting. However, the relationship between
herbivore species richness and altitude has not been examined in detail. We show
that while the overall regional pattern (gamma-richness) for 25 large-herbivore
species (56 % grazers, 44 % browsers/mixed feeders) in the Western Himalayas
shows a mid-elevation peak, the species richness of grazers increases nearly
monotonically with altitude peaking at 4000–5000 m. Median body mass of herbivores decreased with altitude, suggesting greater suitability of higher elevations
for smaller bodied herbivores. We propose that seasonal altitudinal migration
patterns, biogeographic influences, increases in the abundance of graminoids, and
an increase in plant nutrients with altitude might explain the unusual high grazer
species richness at higher altitudes in the Himalayan Mountains.

Journal Article

2014

A penny saved is a penny earned: lean season foraging strategy of an alpine ungulate